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Saturday, June 9, 2012

Spent Fuel Pool Containment: Guest Post

Recently, I wrote an article with help from many other people which was presented on the ANS Nuclear Cafe in order to refute misinformation about the safety of spent fuel at Fukushima Daiichi. There was some material that did not make the final edit, for space as well as focus related reasons.

Ted DelGaizo submitted a piece which was in response to statements by Mr. Markey and others on 'loss of containment' of spent fuel pools. With the very recent news this week surrounding spent fuel safety on nuclear plant sites, and the future of a spent fuel repository, I think it is important that his piece get published now. Below is the piece Ted submitted in its entirety; following the piece is a short bio.

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RADIOLOGICAL CONTAINMENT COMPARISONS

Ted DelGaizo

In a potential reactor accident, radioactive materials must be contained and release to the environment must be prevented. The reactor containment is designed for this purpose. In a spent fuel pool with a loss of cooling, the ultimate fuel-cooling is through the mechanism of pool boiling. The boiling pool steam is not contained but is released to the atmosphere. Hence, the concept of containment does not apply to spent fuel pool cooling other than indirectly by maintaining fuel integrity through pool boiling.

The reactor plants that comprise the current fleet of nuclear-electric generating plants in the USA are primarily pressurized water reactors or boiling water reactors. In either case, the reactor core is cooled by tens of thousands of gallons of water at temperatures typically at 500°F and above. In the unlikely event of a cooling-system piping-rupture, the water would be released to the surrounding (containment) building and would immediately flash to steam because the containment building is initially at or near atmospheric pressure. The large volume of steam would pressurize the containment building above atmospheric pressure, typically to pressures in the vicinity of 30 to 50 pounds per square inch (gauge pressure).

Since the containment building would become pressurized in the loss-of-coolant accident postulated above, the containment must be capable of withstanding these pressures in order to preclude escape (to the outside environment) of the steam and any entrained radioactive gasses, particles, and other radioactive materials that would also be released to the containment atmosphere. Consequently, reactor containment buildings are generally constructed of several foot-thick concrete walls with reinforcing steel bars and with stainless-steel liners to completely contain the internal atmosphere.

These large and extremely strong buildings also protect the reactor and other internal equipment from external hazards such as tornadoes, hurricanes, and even aircraft impacts. The reactor building at the Chernobyl plant outside Kiev in the Ukraine was not constructed with this USA-style containment building and consequently substantial levels of radioactive materials escaped into the outside environment following the accident there in 1986.

Reactor containments are not needed for spent fuel pools. Spent fuel pools are large swimming-pool type structures open to atmospheric pressure. The pools are typically 35 to 40 feet deep, with the spent fuel sitting in steel racks at the bottom of the pools and a minimum of about 25 feet of water sitting above the top of the fuel racks. The water is essentially clean and contains some minimal radioactive contamination that leaches from the fuel but is constantly being filtered and purified by a filtration and cooling system.

If the forced cooling system fails and cannot be reestablished for a sustained time period (e.g. due to a total loss of electric power as occurred at Fukushima), the pool will slowly heat to 212°F and will eventually boil. A boiling pool is a safe pool from a nuclear or radioactive standpoint as long as the pool water that evaporates or boils from the surface is replaced such that the boiling (which is removing heat from the fuel) can continue indefinitely or until forced cooling is reestablished. Boiling rates would typically require less than 100 gallons per minute of replacement water that can normally be supplied from a variety of sources including gravity flow from make-up tanks or via make up pumps that can be supplied by diesel-engine driven pumps (such as fire-water pumps or fire trucks) even when site electrical power remains unavailable.

While spent fuel pools normally sit in buildings, the buildings themselves are unnecessary in order to capture or retain the boiling steam because the steam must be vented into the outside atmosphere in any case. Steam from a spent fuel pool may contain some minimal level of radioactive contamination that will be so low as to be essentially undetectable by current monitoring equipment and would pose no threat to the general public. There are no comparisons between the steam released from a boiling spent fuel pool and the radioactive materials (fission gases and other highly radioactive materials) that would be found in the atmosphere of a reactor containment building following a major loss-of-coolant accident. They are orders of magnitude different and should not in any way be considered as equivalent from a radiological or other hazard standpoint. Hence, comparisons of containments for reactors and containments for spent fuel pools are equally illegitimate.

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Mr. DelGaizo is a 1963 graduate of the US Naval Academy and has over forty-five years of experience in nuclear-power engineering, including plant operations, design, licensing, modification, and quality assurance. He has been a technical consultant to the USNRC and a consultant to senior nuclear utility management. Highlights of his professional career include nuclear plant operation as a US Navy submarine officer including duty as chief engineer officer and submarine executive officer; nuclear-mechanical design-engineer with a major architect-engineer firm; and an emergency response team member at Three Mile Island Unit 2 following the 1979 reactor accident.